This invention relates to a rear-projection type projection display apparatus that projects a picture from the rear of a screen, and allows it to be viewed from the front of the screen.
A projection display apparatus having a mechanism for projecting a bright picture for viewing by a viewer not directly in front of a screen is disclosed, for example, in Japanese Utility Model Kokai Publication No. 116303/1986, Japanese Utility Model Kokai Publication No. 28068/1989 and Japanese Patent Kokai Publication No. 196001/1989. According to the apparatus described in these publications, a bright picture is presented to a viewer looking down on the screen from above or up at the screen from below by displacing a Fresnel lens up and down, this Fresnel lens being moved either independently or in conjunction with a diffusing plate.
However, according to the aforementioned conventional projection display apparatus, the Fresnel lens can move only up and down, and cannot move to the left and right. A bright picture, therefore, could not be presented to a viewer standing to the left or right of the screen.
Moreover, in the aforesaid conventional projection display apparatus, no means was provided to indicate the direction in or the extent to which the Fresnel lens should be displaced, and it was sometimes troublesome to make the adjustments necessary to move it to the optimum position.